This is a good story, and you handled everything quite well. I did have to gave up trying to find all the endings: too hard and time consuming. I also thought it would be longer, but it is more an epic story than epic length I think (compared to number of areas in The Machine for instance).
Is Basalt Woods tileset usable? It so beautiful I'd really like to use it. Below is my favorite screen by the way. The way you used the darker waterfall to imply depth was brilliant. I don't think I've seen that done before.
To understand why it's epic length, you will need to hunt down one of the bigger game secrets. To do this, you need to get all 4 keys and use them. If you have to cheat to find this part of the game, then so be it.
Also, you may use any tileset in the level that doesn't say otherwise, and Basalt Woods is one of those you may use. If you want to use the tilesets in there that mention being made specifically for Purple Ink, you will need to contact that user for permission. I'm sure Purple Ink will let you do so if you ask.